Optical scanning unit and image forming apparatus

ABSTRACT

An optical scanning unit irradiates a polygon mirror with a plurality of light beams emitted from a light source according to image data, and scans and exposes a plurality of photoreceptors with the plurality of light beams as scanning lights. The optical scanning unit comprises a primary optical system unit including an optical system that emits the plurality of light beams, which are emitted from the light source toward the polygon mirror, and a secondary optical system unit including an optical system that emits the light beams, which are reflected by the polygon mirror toward the photoreceptors. The arrangement is such that the primary optical system unit is fitted removably to the secondary optical system unit, and that a combined unit of the primary optical system unit and the secondary optical system unit is removably fitted to an image forming apparatus having the photoreceptors.

CROSS-NOTING PARAGRAPH

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2005-146553 filed in JAPAN on May 19, 2005,the entire contents of which are hereby incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to an optical scanning unit andan image forming apparatus equipped with the optical scanning unit, and,more particularly, to an optical scanning unit used forelectrophotographic image forming apparatuses such as digital copymachines, printers, and facsimile machines, for example.

BACKGROUND OF THE INVENTION

Image forming apparatuses such as digital copy machines, laser printers,or facsimile machines are widely used. Such an image forming apparatususes an optical scanning unit that scans a laser beam. When an image isformed in the image forming apparatus, after a photoreceptor is chargedby a charging unit, an electrostatic latent image is formed on thephotoreceptor by the optical scanning unit by writing according to imageinformation. The electrostatic latent image on the photoreceptor isdeveloped by toner supplied from a development unit. The toner imagedeveloped on the photoreceptor is transferred to recording paper by atransfer unit and is fixed on the recording paper by a fixing unit toobtain a desired image.

Along with speeding up of color image forming apparatuses such asdigital copy machines and laser printers, a tandem-mode apparatus is inpractical use which includes a plurality of photoreceptors in tandemarrangement. In this case, for example, four photoreceptor drums arearranged in the conveying direction of the recording paper; eachphotoreceptor is simultaneously exposed to light by a scanning opticalsystem corresponding to each photoreceptor drum to form a latent image;and these latent images are developed by development units that usedevelopers with different colors such as yellow, magenta, cyan, andblack. The developed toner images are sequentially transferred on thesame recording paper in an overlapping manner to obtain a color image.

As compared to a mode that forms each color image with one photoreceptorsequentially, since the tandem mode exposing a plurality ofphotoreceptors simultaneously can output color and monochrome images atthe same speed, the tandem mode is advantageous in high-speed printing.On the other hand, since scanning optical systems corresponding to aplurality of photoreceptors are necessary, a unit for exposing thephotoreceptors tends to be large in size and the challenge is tominiaturize the unit. Another challenge is to eliminate color drift whenthe toner image developed on each photoreceptor is transferred to therecording paper in an overlapping manner.

With regard to the tandem-mode image forming apparatus as describedabove, for example, Japanese Laid-Open Patent Publication Nos.2004-109700 and 2004-109699 disclose an optical scanning unit thatincludes a wedge-shaped prism disposed on an optical path from a lightsource unit to deflecting means and a writing start position correctingmeans for rotating and adjusting the wedge-shaped prism around anoptical axis to vary a position of a beam spot in the sub-scanningdirection, and the optical scanning unit can control the beam spotposition on a photoreceptor drum during the writing of image data. Atthe time of continuous printing, relative color drift of each color canbe corrected effectively to output a good color image with less colordrift.

Japanese Laid-Open Patent Publication No. 2004-233638 discloses a lensadjustment apparatus that includes: a light source; a device thatdivides the light emitted from the light source into four light beams;an adjustment device that drives a second lens on a plane where a normalline is the optical axis of the second lens; a diffraction grating thatdiffracts the collected light beams from the first and second lenses togenerate interference; a micromotion stage that drives the diffractiongrating in the direction including a component of a direction verticalto the groove direction of the grating plane; four interference imageobservation systems that are constituted by an objective lens, animaging lens, and a CCD camera to observe the interference light; aprocessing device that processes the four interference images to detectone aberration from aberrations sensitive to the decentering of thesecond lens; and a control device that detects an adjustment amount fromthe detected aberration to drive the adjustment device, and the lensadjustment apparatus can adjust a lens with small NA highly accurately.

Japanese Laid-Open Patent Publication No. 2002-90675 discloses anoptical scanning unit that includes: an optical deflection device thatdeflects light in a predetermined direction; a plurality of laserelements; a pre-deflection optical system that includes a glass lens anda plastic lens to convert a cross-sectional shape of light emitted fromeach laser element into a predetermined shape; and a post-deflectionoptical system that includes two lenses forming an image such that eachof the light deflected by the optical deflection device is scanned on apredetermined image plane at a constant speed. The power of the twolenses of the deflection optical systems is regulated to be positiverelative to the direction orthogonal to the rotation axis of thereflection face of the optical deflection device. At least one of thelens surfaces of each of the lenses is formed to be a lens withoutrotational symmetric surfaces. In this way, an optical scanning unit canbe provided which is suitable for an image forming apparatus that canprovide a color image without color drift at low cost.

In the image forming apparatuses as described above, specifications ofprint speed (image forming speed) are different depending on the model.For example, some models can print 20 or more sheets per minute andother models can print 60 or more sheets per minute. As the print speedis desired to be increased, it is desirable that the specification ischanged from low-speed to high-speed by changing optical elements,operation conditions, etc. of an image forming apparatus.

When such a change in the specification is performed in the imageforming apparatus, in the optical scanning unit, the specification ofthe optical element must be changed or an optical path must be adjustedby changing the arrangement. The optical scanning unit itself must beconfigured to be removable from the image forming apparatus and, whenthe optical scanning unit is incorporated into the image formingapparatus, the optical scanning unit must be able to be incorporatedwhile maintaining the installation accuracy.

However, conventionally, an optical scanning unit has not been proposedwhich facilitates the highly accurate installation to an image formingapparatus or which facilitates the highly accurate replacement of theoptical element, change in the optical path, etc. in preparation for thespecification changes such as the speeding up as described above.

SUMMARY OF THE INVENTION

An object of the present invention to provide an optical scanning unitthat can support the specification changes such as the speeding up ofimage formation by replacing the optical elements of the optical systemsor by changing their arrangement and that is configured to beexchangeable such that the optical scanning unit and the optical elementthereof can be shared by a plurality of models or a plurality ofspecifications of an image forming apparatus. And another object of thepresent invention is to provide an image forming apparatus equipped withsuch an optical scanning unit.

Another object of the present invention is to provide an opticalscanning unit that irradiates a polygon mirror with a plurality of lightbeams emitted from a light source according to image data, that convertsthe plurality of light beams to scanning lights by the rotation of thepolygon mirror, and that scans and exposes a plurality of photoreceptorssimultaneously with the plurality of scanning lights to form latentimages on the respective photoreceptors, where the optical scanning unitcomprises a primary optical system unit including the light source andan optical system that emits the plurality of light beams emitted fromthe light source toward the polygon mirror; and a secondary opticalsystem unit including the polygon mirror and an optical system thatemits the light beams reflected by the polygon mirror toward thephotoreceptors, wherein the primary optical system unit is fittedremovably to the secondary optical system unit and wherein a combinedunit of the primary optical system unit and the secondary optical systemunit is fitted removably to an image forming apparatus comprising thephotoreceptors.

Another object of the present invention is to provide the opticalscanning unit wherein the primary optical system unit includes a firstlaser diode, a second laser diode, a third laser diode, and a fourthlaser diode, which act as the light source, wherein the primary opticalsystem unit further includes a first mirror that reflects the lightbeams emitted from the second to fourth laser diodes; a second mirrorthat reflects the light beam emitted from the first laser diode and thelight beams reflected by the first mirror; a primary optical systemcylindrical lens that acts on the light beams emitted from the secondmirror; and a third mirror that reflects the light beams emitted fromthe primary optical system cylindrical lens toward the polygon mirror,and wherein the secondary optical system unit includes the polygonmirror, an fθ lens that acts on the light beams emitted from the polygonmirror, and a secondary optical system cylindrical lens that makes thelight beams emitted from the fθ lens converge on the surfaces of thephotoreceptors.

Another object of the present invention is to provide the opticalscanning unit wherein the scanning direction on the surface of thephotoreceptor is defined to be a main scanning direction and the arraydirection of the plurality of photoreceptors is defined to be asub-scanning direction, wherein the fθ lens is constituted by a first fθlens and a second fθ lens, wherein the individual light beam emittedfrom the polygon mirror is parallel light in the main scanning directionas well as diffused light in the sub-scanning direction with eachoptical axis of the plurality of light beams traveling in thesub-scanning direction in a diffusing manner forming an angle relativeto each optical axis, wherein the first fθ lens and the second fθ lensconvert the individual light beam of parallel light in the main scanningdirection to convergent light approximately converging on the surface ofthe photoreceptor, wherein the second fθ lens converts the plurality oflight beams diffusing in the sub-scanning direction such that theoptical axes of the plurality of light beams become parallel to eachother, and wherein the second optical system cylindrical lens acts onlyon the sub-scanning direction of the light beams emitted from the secondfθ lens to convert the light beams including the individual light beamthat is parallel light and the plurality of light beams which opticalaxes are parallel to each other such that the individual light beamapproximately converges on the surface of the photoreceptor as well assuch that the plurality of light beams approximately converges on thesurfaces of the photoreceptors.

Another object of the present invention is to provide the opticalscanning unit wherein the secondary optical system unit is configured bydisposing a plurality of optical elements including the polygon mirrorwithin a chassis and wherein the primary optical system unit is loadedto the secondary optical system unit from the bottom side of the chassiswhere the optical elements are disposed.

Another object of the present invention is to provide the opticalscanning unit wherein at least a part of the optical elementsconstituting the optical systems of the primary optical system unit andthe secondary optical system unit is configured to be removable suchthat reassembling can be performed depending on the performance of animage forming apparatus comprising the photoreceptors.

Another object of the present invention is to provide the opticalscanning unit wherein the primary optical system unit and/or thesecondary optical system unit include(s) a positioning mechanism forchanging the installation positions of the optical elements forreassembling depending on the performance of an image forming apparatusand wherein when the optical elements are reassembled, unnecessaryoptical elements can be removed to dispose necessary optical elements atthe positioning mechanism.

Another object of the present invention is to provide the opticalscanning unit wherein at least a part of the optical elementsconstituting the optical systems of the primary optical system unit andthe secondary optical system unit is configured to be adjustable in anangle or a position relative to the incident light beams.

Another object of the present invention is to provide the opticalscanning unit wherein the adjustable optical elements include the secondmirror, the third mirror, and the secondary optical system cylindricallens.

Another object of the present invention is to provide the opticalscanning unit wherein an adjusting mechanism for adjusting the angle orposition is configured so as to be operated from one side of eachoptical system unit.

Another object of the present invention is to provide the opticalscanning unit wherein the one side of each optical system unit is theoperational side of an image forming apparatus to which the opticalsystem units are attached.

Another object of the present invention is to provide the opticalscanning unit wherein a chassis of the secondary optical system unitincludes two fixing shafts and wherein the combined unit of the primaryoptical system unit and the secondary optical system unit is fixed tothe image forming apparatus by the fixing shafts.

Another object of the present invention is to provide the opticalscanning unit wherein a chassis of the optical scanning unit issupported, with respect to the vertical direction, at three points ofthe fixing shafts and fixed to the fixing shafts.

Further, another object of the present invention is to provide an imageforming apparatus comprising the photoreceptors and the optical scanningunit, wherein the optical scanning unit forms latent images on thephotoreceptors, the latent images being developed for image formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a configuration example of an image forming apparatus usingan optical scanning unit of the present invention.

FIG. 2 is a plan view of a configuration example of a primary opticalsystem unit of the optical scanning unit of the present invention.

FIG. 3 is a perspective schematic diagram of the primary optical systemunit of FIG. 2.

FIGS. 4A and 4B show a configuration example of a secondary opticalsystem of the optical scanning unit.

FIGS. 5A and 5B are diagrams for describing the state of the individuallight beam of each color in the primary optical system and the secondaryoptical system.

FIG. 6 shows optical paths of four light beams in the sub-scanningdirection schematically.

FIG. 7 shows a configuration of the BD sensor detecting the writingposition in the sub-scanning direction of the light beam schematically.

FIGS. 8A to 8C are diagrams for describing a configuration example ofthe BD sensor that can be applied to the embodiment.

FIG. 9 is a diagram for describing a configuration example of installinga mask to increase the detection accuracy of the BD sensor.

FIG. 10 is a block diagram for describing a configuration example of acontrol system of the optical scanning unit.

FIG. 11 is a diagram for describing an angle adjustment mechanism of acylindrical lens of the secondary optical system.

FIG. 12 is a relevant part enlarged schematic diagram for describing anadjustment mechanism for adjusting the angle of the cylindrical lens ofthe secondary optical system.

FIGS. 13A and 13B are diagrams for describing a shaft configuration forfixing the optical scanning unit within the image forming apparatus at apredetermined position.

FIGS. 14A to 14D are diagrams for describing a mechanism that fixes afirst fixing shaft 221 a.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a configuration example of an image forming apparatus usingan optical scanning unit of the present invention. The image formingapparatus forms a multicolor or monochrome image on a predeterminedsheet (recording paper) depending on image data transmitted fromoutside. As shown in FIG. 1, the image forming apparatus is constitutedby an exposure unit 1, development units 2, photoreceptor drums 3,cleaner units 4, charging units 5, an intermediate transfer belt unit 6,a fixing unit 7, a paper feeding cassette 8, a paper ejection tray 9,etc.

The image data handled in the image forming apparatus correspond to acolor image using colors of black (K), cyan (C), magenta (M), and yellow(Y). Therefore, four sets of the development units 2, the photoreceptordrums 3, the charging units 5, and the cleaner units 4 are provided suchthat four types of latent images according to respective four colors areformed and each set is set to black, cyan, magenta, or yellow toconstitute four image stations.

The charging unit 5 is charging means for electrostatically charging thesurface of the photoreceptor drum 3 uniformly to a predeterminedelectric potential and, in addition to the contact type such as rollertype or brush type as shown in FIG. 1, a charger type charging unit maybe used.

The exposure unit 1 corresponds to the optical scanning unit relating tothe present invention and is configured to be a laser scanning unit(LSU) equipped with a laser irradiating portion and a reflecting mirroras shown in FIG. 1. The exposure unit 1 is provided with a polygonmirror 201 that scans laser beams, and optical elements such as lensesand mirrors for guiding the light beams reflected by the polygon mirror201 to the photoreceptor drums 3. The configuration of the opticalscanning unit constituting the exposure unit 1 will be described indetail later. In some techniques, for example, the exposure unit 1 maybe an EL or LED writing head where light emitting elements are arrangedin an array.

The exposure unit 1 has a function for exposing the chargedphotoreceptor drums 3 according to the input image data to formelectrostatic latent images corresponding to the image data on thesurfaces of the photoreceptor drums 3. The development unit 2 developsthe electrostatic latent image formed on each photoreceptor drum 3 withtoner of each of four colors (Y, M, C, and K). The cleaner unit 4removes and collects the toner remaining on the surface of thephotoreceptor drum 3 after the development and the image transfer.

The intermediate transfer belt unit 6 is disposed above thephotoreceptor drums 3 and includes an intermediate transfer belt 61, anintermediate transfer belt driving roller 62, an intermediate transferbelt tension mechanism 63, an intermediate transfer belt driven roller64, intermediate transfer rollers 65, and an intermediate transfer beltcleaning unit 66. Four intermediate transfer rollers 65 are provided forrespective four colors of Y, M, C, and K.

The intermediate transfer belt 61 is laid with tension and isrotationally driven in the direction indicated by an arrow M by theintermediate transfer belt driving roller 62, the intermediate transferbelt tension mechanism 63, the intermediate transfer belt driven roller64, and the intermediate transfer rollers 65. Each intermediate transferroller 65 is rotatably supported by an intermediate transfer rollerattachment portion of the intermediate transfer belt tension mechanism63 of the intermediate transfer belt unit 6 and gives a transfer biasfor transferring the toner image on the photoreceptor drum 3 onto theintermediate transfer belt 61.

The intermediate transfer belt 61 is provided so as to contact with eachphotoreceptor drum 3. The intermediate transfer belt 61 has a functionfor forming a color toner image (multicolor toner image) on theintermediate transfer belt 61 by transferring the toner image of eachcolor formed on the photoreceptor drum 3 sequentially onto theintermediate transfer belt 61 in an overlapping manner. The intermediatetransfer belt 61 is formed using a film with a thickness of about 100 μmto 150 μm so as to have no end.

The transfer of the toner image from the photoreceptor drum 3 to theintermediate transfer belt 61 is performed by the intermediate transferroller 65 in contact with the back side of the intermediate transferbelt 61. To the intermediate transfer roller 65, a high-voltage transferbias (high voltage with the opposite polarity (+) to the chargingpolarity (−) of the toner) is applied for transferring the toner image.The intermediate transfer roller 65 is a roller having a base of a metal(e.g., stainless steel) shaft with a diameter of 8 to 10 mm and thesurface of the shaft is covered with a conductive elastic material(e.g., EPDM, urethane foam, etc.). With the conductive elastic material,the high voltage can be applied uniformly to the intermediate transferbelt 61. Although the roller shape is used for the transfer electrode inthis embodiment, a brush may be used.

The electrostatic image is developed on each photoreceptor drum 3correspondingly to each color as described above and is laminated on theintermediate transfer belt 61. With the rotation of the intermediatetransfer belt 61, the laminated image information is transferred ontopaper by a transfer roller 10 (described later) disposed at a contactposition between paper and the intermediate transfer belt 61.

The intermediate transfer belt 61 and the transfer roller 10 are pressedagainst each other with a predetermined nip and a voltage (high voltagewith the opposite polarity (+) to the charging polarity (−) of thetoner) is applied to the transfer roller 10 to transfer the toner topaper. To obtain the nip constantly with the transfer roller 10, one ofthe transfer roller 10 and the intermediate transfer belt driving roller62 is made of a hard material (e.g., metal) and the other utilizes asoft material roller such as an elastic roller (e.g., elastic rubberroller or resin foam roller).

Since color mixture at the next procedure is generated by the tonerattached to the intermediate transfer belt 61 by contacting with thephotoreceptor drum 3 or by the toner that is not transferred onto paperby the transfer roller 10 and remained on the intermediate transfer belt61, the toner is configured to be removed and collected by theintermediate transfer belt cleaning unit 66. The intermediate transferbelt cleaning unit 66 includes, for example, a cleaning blade that is acleaning member contacting with the intermediate transfer belt 61 andthe intermediate transfer belt 61 contacting with the cleaning blade issupported by the intermediate transfer belt driven roller 64 from theback side.

The paper feeding cassette 8 is a tray for storing sheets (recordingpaper) used for forming images and is provided on the under side of theexposure unit 1 of the image forming apparatus. The paper ejection tray9 is provided on the top side of the image forming apparatus and is atray for accumulating the printed sheets face-down.

The image forming apparatus is provided with a paper conveying path S ina substantially vertical shape for sending the sheets in the paperfeeding cassette 8 to the paper ejection tray 9 via the transfer roller10 and the fixing unit 7. A pickup roller 11, a plurality of conveyingrollers 12 a to 12 e, a resist roller 13, the transfer roller 10, thefixing unit 7, etc. are disposed near the paper conveying path S fromthe paper feeding cassette 8 to the paper ejection tray 9.

The conveying rollers 12 a to 12 e are small rollers for facilitatingand aiding the conveyance of the sheets and are provided along with thepaper conveying path S. The pickup roller 11 is provided near the end ofthe paper feeding cassette 8 and picks up the sheets one-by-one from thepaper feeding cassette 8 to supply the sheets to the paper conveyingpath S.

The resist roller 13 holds the sheet conveyed through the paperconveying path S temporarily. The resist roller 13 has a function forconveying the sheet to the transfer roller 10 at the timing when theleading end of the toner image on the photoreceptor drums 3 is matchedwith the leading end of the sheet.

The fixing unit 7 includes a heat roller 71 and a pressure roller 72,and the heat roller 71 and the pressure roller 72 hold the sheet and arerotated. The heat roller 71 is set to be a predetermined fixingtemperature by a controlling portion based on a signal from atemperature detector not shown and performs the thermocompression of thetoner to the sheet along with the pressure roller 72 to melt/mix/pressthe multicolor toner image transferred to the sheet to thermally fix theimage to the sheet.

Detailed description will be made of the sheet conveying path. Asdescribed above, the image forming apparatus is provided with the paperfeeding cassette 8 preliminary storing the sheets. To feed the sheetsfrom the paper feeding cassette 8, the pickup roller 11 is disposed toguide the sheets to the conveying path S one-by-one.

The sheet fed from the paper feeding cassette 8 is conveyed to theresist roller 13 by a conveying roller 12 a on the paper conveying pathS and is conveyed to the transfer roller 10 at the timing of matchingaccurately the leading end of the sheet with the leading end of theimage information on the intermediate transfer belt 61, and the imageinformation is written onto the sheet. Subsequently, when the sheetpasses through the fixing unit 7, the unfixed toner on the sheet isthermally melted and fixed and the sheet passes through a conveyingroller 12 c disposed behind the fixing unit 7 and is ejected on thepaper ejection tray 9.

The above conveying path is used when one-side printing is requested forthe sheet and, when two-side printing is requested, after the one-sideprinting is completed as described above and the rear end of the sheetwhich has passed through the fixing unit 7 is chucked by the finalconveying roller 12 c, the conveying roller 12 c is rotated reversely toguide the sheet to conveying rollers 12 d, 12 e. Subsequently, after thesheet passes through the resist roller 13 and the back side of the sheetis printed, the sheet is ejected on the paper ejection tray 9.

Specific description will be made of the embodiment of the opticalscanning unit of the present invention.

The optical scanning unit of the embodiment can be applied to thetandem-mode image forming apparatus that has a plurality of thephotoreceptor drums 3 as described above to form a color image bysimultaneously scanning and exposing the photoreceptor drums 3 with aplurality of light beams to form images with different colors on therespective photoreceptor drums 3 and by overlapping the images ofrespective colors on the same transfer medium.

As described above, the image forming apparatus is provided with thephotoreceptor drum for forming a black (K) image, the photoreceptor drumfor forming a cyan (C) image, the photoreceptor drum for forming amagenta (M) image, and the photoreceptor drum for forming a yellow (Y)image at substantially even intervals. Since the image of each color isformed simultaneously, the tandem-mode image forming apparatus canreduce the time for forming a color image considerably.

In the following description, K, C, M, and Y stand for black, cyan,magenta, and yellow, respectively.

The optical scanning unit according to the present invention forexposing the photoreceptor drums 3 is constituted by a primary opticalsystem (incoming optical system) and a secondary optical system(outgoing optical system). The primary optical system includes foursemiconductor lasers emitting Y, M, C, and K light beams, respectively,and optical elements such as mirrors and lenses guiding these lightbeams to a polygon mirror 201 (rotational polygon mirror) of thesecondary optical system. The secondary optical system includes thepolygon mirror 201 that scans the laser beams on the photoreceptor drums3, i.e., scanned objects, optical elements such as mirrors and lensesguiding the light beams reflected by the polygon mirror 201 to thephotoreceptor drums 3, and a BD sensor that detects the light beams. Thepolygon mirror 201 is configured to be shared by each color.

FIG. 2 is a plan view of a configuration example of the primary opticalsystem unit of the optical scanning unit of the present invention andFIG. 3 is a perspective schematic diagram of the primary optical systemunit of FIG. 2. In FIGS. 2 and 3, a numeral 100 is the primary opticalsystem unit; a numeral 101 is a laser diode; a numeral 102 is acollimator lens; a numeral 103 is an aperture; a numeral 104 is a laserdrive substrate; a numeral 105 is a laser holder; a numeral 106 is alens holder; a numeral 107 is a body tube; a numeral 108 is a attachmentscrew; a numeral 110 is a first mirror; a numeral 111 is a secondmirror; a numeral 112 is a cylindrical lens; a numeral 113 is a thirdmirror; and a numeral 120 is a base for installing the optical elementsof the primary optical system.

Each of K, C, M, and Y laser diodes 101 is driven by a laser drivecircuit (not shown) that is light source driving means. The laser drivecircuit receives various control signals output from a controllingportion of the image forming apparatus and the image data supplied froman image processing portion and controls the emission of each laserdiode 101 in accordance with these control signals and image data.

On the laser emitting side of each laser diode 101, the collimator lens102 is provided for K, C, M, and Y. The light beam output from eachlaser diode 101 is diffused light in a substantially elliptical shapeand is converted to parallel light by the collimator lens 102 providedfor each color. After the collimator lens 102 for each color, anaperture (slit) 103 with a predetermined gap is provided to regulate thediameter of the light beam. In this specification, the parallel lightindicates the state that the diameter of the light flux does not changeas the beam travels, and this state is differentiated from the statethat optical axes of a plurality of beams are parallel to each other.

Each laser diode 101 is attached to the laser holder 105. The laserholder 105 is attached to the back side of the lens holder 106 formedintegrally on the base of the primary optical system. The body tube 107provided with the collimator lens 102 and the aperture 103 is providedto the front side of the lens holder 106. The light beam emitted fromthe laser diode 101 exits to the front and outside of the body tube 107through the collimator lens 102 and the aperture 103.

The light beam emitted from the body tube 107 of the K laser diode 101goes to the second mirror 111 through the K collimator lens 102 and theK aperture 103. The light beams emitted from the body tubes 107 of theC, M, and Y laser diodes 101 enter into the first mirror 110 through theC, M, and Y collimator lenses 102 and apertures 103, respectively. Thefirst mirror 110 is constituted by three mirrors individually reflectingthe C, M, and Y light beams, and the light beam of each color reflectedby the mirror travels in the traveling direction of the K light beam andis made incident on the second mirror 111.

The laser diode 101 of each color is disposed at a different height inthe sub-scanning direction (direction vertical to the substratesurface). The difference in the height is set to about 2 mm, forexample. The first mirror 110 is disposed at a position where only thelight beam emitted from the corresponding laser diode 101 is reflected.The three (C, M, and Y) mirrors constituting the first mirror 110 arerespectively disposed at a position overlapping with the light beamemitted from the K laser diode 101 in the main scanning direction.

With the configuration as described above, the K light beam emitted fromthe K laser diode 101 and the C, M, and Y light beams reflected by thefirst mirror 110 are completely matched in the main scanning direction,have the displacement (vertical difference) in the sub-scanningdirection, and are made incident on the second mirror 111 while theoptical axes of these light beams are parallel to each other. The lightbeam of each color emitted from each collimator lens 102 is the parallellight with the diameter of the light flux not changed as the light beamtravels.

The second mirror 111 makes the incident light beam of each color K, C,M, and Y incident on the cylindrical lens 112. The cylindrical lens 112is disposed for focusing the incident light beam of each color in thesub-scanning direction. The light beam of each color emitted from thecylindrical lens 112 is reflected by the third mirror 113 and madeincident on the reflection face of the polygon mirror 201.

The cylindrical lens 112 has a lens power in the sub-scanning directionand is set such that the light beam converges in the vicinity of thereflection face of the polygon mirror 201 in the sub-scanning directiondepending on the optical path length from the cylindrical lens 112 tothe polygon mirror 201. That is, the light beam of each color is madeincident on the cylindrical lens 112 while each light beam is theparallel light and converges approximately at the surface of thereflection face of the polygon mirror 201 in the sub-scanning direction.At the same time, the light beam of each color is made incident on thecylindrical lens 112 while the optical axes are parallel to each otherand converges at approximately the same position of the surface of thepolygon mirror 201.

Since the cylindrical lens 112 does not have a lens power in the mainscanning direction, the incident light beam of each color is emitteddirectly as the parallel light in the main scanning direction and ismade incident on the reflection face of the polygon mirror 201.Typically, in the main scanning direction, the parallel light is madeincident on the polygon mirror 201. The convergent light in the mainscanning direction is not preferable since a negative field curvature isgenerated by the fθ lens described later. In the sub-scanning direction,to correct a face skew of the reflection face, the light beams are madeconvergent at the surface of the reflection face. For example, in thesub-scanning direction, the position of the light beam made incident onthe reflection face of the polygon mirror 201 is in the vicinity of thecenter in the height direction of the reflection face.

In the optical scanning unit of the embodiment, four light beams for Y,M, C, and K are deflected by one polygon mirror 201 of the secondaryoptical system. In this case, the four light beams must be divided afterbeing reflected by the polygon mirror 201 and the displacement in themain scanning direction must not be generated in the light beam for eachcolor. Therefore, the four light beams emitted from the cylindrical lens112 of the primary optical system are set to be made incident on thepolygon mirror 201 at the same position from the same direction in themain scanning direction and are set to be made incident on substantiallythe same position from the directions with angle differences in thesub-scanning direction. Such optical path setting is achieved by thearrangement of the laser diodes 101 with the vertical differences in thesub-scanning direction since all the light beams for the respectivecolors are matched in the main scanning direction and travel withpredetermined vertical differences in the sub-scanning direction.Therefore, the light beam for each color can be divided by the scanningoptical system.

In the above configuration, since the light beam for each color is theparallel light and the optical axes thereof are parallel to each otheron the optical path from the collimator lens 102 to the cylindrical lens112 for each color in the primary optical system, the optical pathlength from the collimator lens 102 to the cylindrical lens 112 can befreely set.

FIGS. 4A and 4B show a configuration example of the secondary opticalsystem of the optical scanning unit; FIG. 4A shows a configurationdiagram of the inside of the chassis of the secondary optical systemunit viewed from above; and FIG. 4B shows a general configuration of theinside of the chassis 223 and the photoreceptor from a lateral view. InFIGS. 4A and 4B, a numeral 200 is a secondary optical system unit; anumeral 201 is a polygon mirror; a numeral 202 is a first fθ lens; anumeral 203 is a second fθ lens; a numeral 204 is a K mirror; a numeral205 is a first C mirror; a numeral 206 is a second C mirror; a numeral207 is a third C mirror; a numeral 208 is a first M mirror; a numeral209 is a second M mirror; a numeral 210 is a first Y mirror; a numeral211 is a second Y mirror; a numeral 212 is a third Y mirror; a numeral213 is a synchronous mirror; a numeral 214 is a BD sensor lens; anumeral 215 is a BD sensor; a numeral 220 is a cylindrical lens for eachcolor; numerals 221 a, 221 b are fixing shafts; a numeral 222 is ainstallation position of the primary optical unit; a numeral 223 is achassis; and a numeral 224 is a frame holding the cylindrical lens.

The polygon mirror 201 has a plurality of (e.g., seven) reflection facesin the rotation direction and is rotationally driven by a polygon motornot shown. The polygon motor is installed in a concave portion on theback side of the chassis 223 where the polygon mirror 201 is installedand a cover is provided for sealing the concave portion. The polygonmotor is provided with fins for heat release. The light beam of eachcolor is emitted from the laser diode 101 of the primary optical system,is reflected by the third mirror 113, is reflected by the reflectionface of the polygon mirror 201 of the secondary optical system, andscans the photoreceptor drum 3 through the subsequent optical elements.

Each laser beam made incident on the polygon mirror 201 with the angledifference in the sub-scanning direction maintains the angle differenceand is divided after passing through the scanning optical systemconstituted by the first fθ lens 202 and the second fθ lens 203.

The first fθ lens 202 has a lens power in the main scanning direction.Therefore, in the main scanning direction, the parallel light beamemitted from the polygon mirror 201 converges to be a predetermined beamdiameter on the surface of the photoreceptor drum 3. The first fθ lens202 has a function that converts the light beam moved in the mainscanning direction at a constant angular speed by the constant angularspeed movement of the polygon mirror 201 such that the light beam moveson a scanning line on the photoreceptor drum 3 at a constant linearspeed.

The second fθ lens 203 has a lens power in the sub-scanning direction.Therefore, in the sub-scanning direction, the diffused light beamemitted from the polygon mirror 201 is converted to the parallel light.The second fθ lens 203 also has a lens power in the main scanningdirection and complements the function of the first fθ lens 202 toenable the control of the beam diameter and the constant linear speedmovement of the beam to be performed accurately.

The first fθ lens 202 and the second fθ lens 203 are made of resin. Toform an aspheric shape for obtaining desirable characteristics of the fθlenses, it is preferable to use a resin material for the fθ lenses.Especially, since the second fθ lens 203 has a lens power in both themain scanning direction and the sub-scanning direction, it is preferableto create the second fθ lens 203 with a resin material to obtain acomplex aspheric shape that realizes this characteristic. The optimumresin material is selected in consideration of transparency,formability, optical elasticity rate, heat resistance, hygroscopicproperty, mechanical strength, cost, etc.

Among four light beams for the respective colors divided by the polygonmirror 201 and passing through the first and second fθ lenses 202, 203,the K light beam passes through the first and second fθ lenses 202, 203,is reflected by the K mirror 204, passes through the K cylindrical lens220, and is made incident on the photoreceptor drum 3 (K). On thephotoreceptor drum 3, drawing is performed on the scanning region.

The divided Y light beam is reflected by the first to third Y mirrors210, 211, 212, passes through the Y cylindrical lens 220, and is madeincident on the photoreceptor drum 3 (Y). Similarly, the divided C lightbeam is reflected by the first to third C mirrors 205, 206, 207, passesthrough the C cylindrical lens 220, and is made incident on thephotoreceptor drum 3 (C). The divided M light beam is reflected by thefirst and second M mirrors 208, 209, passes through the M cylindricallens 220, and is made incident on the photoreceptor drum 3 (M).

In the secondary optical system, the cylindrical lens 220 for each colorhas a lens power in the sub-scanning direction. Therefore, in thesub-scanning direction, the parallel incident light beam converges to bea predetermined beam diameter on the photoreceptor drum 3. In the mainscanning direction, the light beam becomes a convergent light in theaforementioned first fθ lens and converges directly on the photoreceptordrum 3. The cylindrical lens 220 is made of resin. For the longcylindrical lens 220 covering the entire scanning width such as theoptical scanning unit, it is preferred for the lens to be formed as aresin lens.

The charged photoreceptor drum 3 is exposed to the light beam of eachcolor emitted from the cylindrical lens 220 according to the image data.In this way, an electrostatic latent image corresponding to the imagedata is formed on the surface of the photoreceptor drum 3. Eachelectrostatic latent image formed on each photoreceptor drum 3 isdeveloped with toner of Y, M, C, or K by the development unit.

Description will be made of the state of the light beam of each coloramong the optical elements in the aforementioned embodiment in anorganized way. FIGS. 5A and 5B are diagrams for describing the state ofthe individual light beam of each color in the primary optical systemand the secondary optical system; FIG. 5A shows a shape of one lightbeam in the main scanning direction schematically; and FIG. 5B shows ashape of one light beam in the sub-scanning direction schematically.

Description will be made of the behavior of the light beam in the mainscanning direction shown in FIG. 5A. The light beam emitted from thelaser diode 101 of the primary optical system is the diffused light andis made incident on the collimator lens 102. In the main scanningdirection, the angle of the diffused light from the laser diode 101 isabout 30 degrees.

The collimator lens 102 converts the incident diffused light to theparallel light, which is emitted. The aperture 103 is provided after thecollimator lens 102, and the diameter of the light beam is regulated bythe opening of the aperture 103. The opening diameter of the aperture103 in the main scanning direction is about 7 mm in this case.

The light beam of the parallel light emitted from the aperture 103 isreflected by the first mirror 110 and the second mirror 111 (only thesecond mirror 111 for K) (not shown in FIG. 5A) and is made incident onthe cylindrical lens 112 of the primary optical system. Since thecylindrical lens 112 of the primary optical system does not have a lenspower in the main scanning direction, the incident parallel light passesthrough without change.

The light beam of the parallel light emitted from the cylindrical lens112 is reflected by the third mirror 113 (not shown in FIG. 5A) and ismade incident on the reflection face of the polygon mirror 201. As shownin the figure, the reflection face of the polygon mirror 201 changes itsangle in the main scanning direction along with the rotation of thepolygon mirror 201.

The light beams of the parallel light reflected by the polygon mirror201 move in the main scanning direction at a constant angular speed, aremade incident on the first fθ lens 202, and are then made incident onthe second fθ lens 203. The first fθ lens 202 and the second fθ lens 203have a lens power in the main scanning direction and convert theparallel incident light beam to the convergent light converging on thesurface of the photoreceptor drum 3. The light beams moving in the mainscanning direction at a constant angular speed are converted such thatthe light beams move on the scanning line on the photoreceptor drum 3 ata constant linear speed.

The second fθ lens 203 is a lens that complements the first fθ lens 202and corrects the light beam emitted from the first fθ lens 202 such thatthe light beam behaves as intended.

The optical path between the second fθ lens 203 and the photoreceptordrum 3 is provided with the mirror(s) (one or a plurality of mirrors foreach color) (not shown in FIG. 5A) for folding and guiding the opticalpath of each color to the target photoreceptor drum 3 and thecylindrical lens 220 of the secondary optical system. Since thecylindrical lens 220 does not have a lens power in the main scanningdirection, the light beam emitted from the second fθ lens 203 is notaffected in the main scanning direction and travels to the photoreceptordrum 3. On the photoreceptor drum 3, the spot diameter of the light beamin the main scanning direction is about 60 μm.

Description will be made of the behavior of the light beam in thesub-scanning direction shown in FIG. 5B. The light beam emitted from thelaser diode 101 is the diffused light and is made incident on thecollimator lens 102, as is the case with the main scanning direction.However, in the sub-scanning direction, the angle of the diffused lightfrom the laser diode 101 is about 11 degrees, which is smaller than thatin the main scanning direction.

The collimator lens 102 converts the incident diffused light to theparallel light, which is emitted. The aperture 103 is provided after thecollimator lens 102, and the diameter of the light beam is regulated bythe opening of the aperture 103. The opening diameter of the aperture103 is about 2 mm in this case.

The light beam of the parallel light emitted from the aperture 103 isreflected by the first mirror 110 and the second mirror 111 (only thesecond mirror 111 for K) (not shown in FIG. 5B) and is made incident onthe cylindrical lens 112 of the primary optical system. Since thecylindrical lens 112 of the primary optical system has a lens power inthe sub-scanning direction, the incident parallel light is converted tothe convergent light that approximately converges on the reflection faceof the polygon mirror 201. The light beam of the parallel light emittedfrom the cylindrical lens 112 is reflected by the third mirror 113 (notshown in FIG. 5B) and is made incident on the reflection face of thepolygon mirror 201. In the sub-scanning direction, the light beamconverges at approximately the center of the reflection face in theheight direction. The face skew of the reflection face is corrected bygenerating a conjugate relationship between the reflection face of thepolygon mirror 201 and the surface of the photoreceptor drum 3.

The light beam reflected by the polygon mirror 201 becomes the diffusedlight, is made incident on the first fθ lens 202, and is then madeincident on the second fθ lens 203. Since the first fθ lens 202 does nothave a lens power in the sub-scanning direction, the light beam of thediffused light made incident on the first fθ lens 202 passes throughwithout change.

The second fθ lens 203 has a lens power in the sub-scanning directionand converts the incident diffused light beam to the parallel light inthe sub-scanning direction.

The optical path between the second fθ lens 203 and the photoreceptordrum 3 is provided with the mirror(s) (one or a plurality of mirrors foreach color) (not shown in FIG. 5B) for folding and guiding the opticalpath of each color to the target photoreceptor drum 3 and thecylindrical lens 220 of the secondary optical system. The cylindricallens 220 has a lens power in the sub-scanning direction, and the lightbeam of the parallel light emitted from the second fθ lens 203 isconverted to the light approximately converging on the surface of thephotoreceptor drum 3. On the photoreceptor drum 3, the spot diameter ofthe light beam in the sub-scanning direction is about 67 μm.

FIG. 6 shows optical paths of four light beams in the sub-scanningdirection schematically. With regard to the optical paths of the lightbeams for the four colors Y, M, C, and K, as described above, althoughthe four light beams travels through the same position in the mainscanning direction, in the sub-scanning direction, the four light beamsemitted from the laser diodes 101 are apart from each other by theheight differences of the laser diodes 101.

As shown in FIG. 6, the four light beams are emitted from the four laserdiodes 101 (for Y, M, C, and K), pass through the collimator lenses 102,and are made incident on the cylindrical lens 112 of the primary opticalsystem with the optical axes thereof parallel to each other. Thecylindrical lens 112 converts each of the four light beams such that thelight beams converge at approximately the center of the reflection faceof the polygon mirror 201. That is, in the sub-scanning direction, thefour light beams converge at approximately the same position on thereflection face of the polygon mirror 201 with angle differences to eachother. In the main scanning direction, the four beams are made incidentat the same position on the reflection face of the polygon mirror 201from the same direction. In FIG. 6, the first to third mirrors 110 to113 are not shown.

The four light beams reflected by the polygon mirror 201 are diffusedagain with angle differences to each other and made incident on thesecond fθ lens 203 via the first fθ lens 202. Since the first fθ lens202 does not have a lens power in the sub-scanning direction, the fourlight beams made incident on the first fθ lens 202 pass through withoutchange. The second fθ lens 203 has a lens power in the sub-scanningdirection and converts the four incident light beams such that theoptical axes thereof become parallel to each other.

The optical path between the second fθ lens 203 and the photoreceptordrum 3 is provided with the mirror(s) (one or a plurality of mirrors foreach color) (not shown in FIG. 6) for folding the optical path of eachcolor and guiding it to the target photoreceptor drum 3, and thesemirrors utilize the displacements of the optical axes of the four lightbeams emitted from the second fθ lens 203 to separate and guide the fourbeams to the respective target photoreceptor drums 3. The lengths of theoptical paths between the second fθ lens 203 and the cylindrical lenses220 in the secondary optical system are the same for all the four lightbeams for the respective colors.

Description will be made of an installation example of a BD (BeamDetect) sensor for detecting the light beam to generate a referencesignal for starting writing before the start of the main scanning of thelight beam on the photoreceptor drum 3.

Among the light beams reflected by the polygon mirror 201 toward thephotoreceptor drum 3, the light beam used for forming an image on thephotoreceptor drum 3, i.e., the light beam for scanning a main scanningline is referred to as a main scanning beam. An image region is definedas a spacial region passed through by the main scanning beam at the timeof the scanning, and a non-image region is defined as a region otherthan the image region.

When the light beam scans the photoreceptor drum 3, the light beam scansthe main scanning line periodically. Since the photoreceptor drum 3 isrotated, the photoreceptor drum 3 is scanned at a different place everycertain period. Every time the scanning of the light beam is performed,the writing start position on the scanning line must be the same.

To detect the writing start position on the scanning line, the opticalscanning unit is provided with a synchronous detector. With reference toFIGS. 4A and 4B, the synchronous detector is constituted by a BD sensor(synchronous detection sensor) 215 for detecting a synchronous detectionbeam that is the light beam in the non-image region, a folding mirror(synchronous mirror) 213 of the synchronous detection beam that isguiding means for guiding the synchronous detection beam to the BDsensor 215, and a BD sensor lens 214 for collecting the synchronousdetection beam to the BD sensor 215.

The synchronous detection beam is a signal for synchronization and isthe light beam which is reflected by the synchronous mirror 213 afterbeing emitted from the polygon mirror 201 and passing through the firstand second fθ lenses 202, 203. The synchronous detection beam is foldedby the synchronous mirror 213 and arrives at the BD sensor 215 via theBD sensor lens 214. The BD sensor 215 outputs a sensor signal dependingon a received light amount. A controlling portion of the opticalscanning unit (e.g., an LSU controller described later) generates asynchronous signal (BD signal) for determining the image writing startposition based on the sensor signal from the BD sensor 215.Specifically, the BD signal is generated if the received light amount ofthe BD sensor 215 is equal to or more than a light amount necessary atleast for exposing the photoreceptor drum 3 with the laser beam to forman electrostatic latent image. The BD signal is used for a scanningstart reference signal in the main scanning direction and the writingstart position on each line is synchronized in the main scanningdirection based on this signal.

The synchronous detector outputs an error signal if the BD sensor 215cannot detect the light beam. The image forming apparatus equipped withthe optical scanning unit stops the operation of the apparatus anddisplays, for example, a predetermined service code on the displayscreen to notify a user of the failure in the writing start position inthe scanning direction.

The BD sensor 215 detecting the writing start position in the scanningdirection is provided only on the optical path of the K light beam tosupport only the K light beam among the four light beams and, withreference to its detection result, the light beams for other colorsstart the scanning at the predetermined writing start timing of theimage data.

In this embodiment, in addition to the BD sensor 215 for detecting thewriting start position in the main scanning direction of the light beam,a BD sensor is provided for detecting the writing position in thesub-scanning direction of each color of Y, M, C, and K. Among the BDsensors for detecting the writing positions in the sub-scanningdirection, the BD sensor for K may be used in conjunction with the BDsensor 215 for detecting the writing start position in the main scanningdirection.

FIG. 7 schematically shows a configuration of the BD sensor detectingthe writing position in the sub-scanning direction of the light beamand, in this figure, a numeral 216 is the BD sensor for detecting thewriting position in the sub-scanning direction; a dotted line aindicates an optical path indicating the image region of the K lightbeam; dotted lines b to e indicate optical paths of light beams for therespective colors in the non-image region. FIG. 7 shows the arrangementof the BD sensors 216 schematically; the elements such as mirrors andlenses are not shown; and the optical paths are simplified. In thecylindrical lens 220, scanning widths are the same for the light beamsfor the colors of Y, M, C, and K.

As shown in FIG. 7, the secondary optical system is provided with fourBD sensors 216 for the respective colors for detecting the writingpositions in the sub-scanning direction. The BD sensors 216 for therespective colors are disposed at positions where the light beams forthe respective colors of Y, M, C, and K in the non-image region can bedetected to determine whether the writing position in the sub-scanningdirection is appropriate or not.

It is preferred to dispose the BD sensors 216 directly in the non-imageforming region on the optical paths of the light beams traveling towardthe photoreceptor drums 3 to achieve a simple configuration. Because ofthe limitation of the space in the non-image region part, if the BDsensors 216 are wished to be disposed at desirable positions in thesecondary optical system, for example, at positions without otheroptical elements, which are out of the optical paths and have spatialmargins, folding mirrors can be employed suitably for the respectivecolors to guide the light beams in the non-image region to therespective BD sensors 216.

As is the case with the BD sensor 215 for detecting the writing startposition in the main scanning direction, the BD sensor 216 for detectingthe writing position in the sub-scanning direction determines whether ornot the writing position in the sub-scanning direction is appropriatefor each light beam, based on whether the light beam can be detected ornot. The optical scanning unit outputs an error signal if the BD sensor216 for each color cannot detect the light beam. The image formingapparatus equipped with the optical scanning unit stops the operation ofthe apparatus and displays, for example, a predetermined service code onthe display screen to notify a user of the failure in the writingposition in the sub-scanning direction. Adjustment will be performedafter verifying the cause such as emission failure in the laser diodes101 for the error-detected colors or misalignment of the mirrors orother optical elements on the optical paths.

The BD sensor 216 for detecting the writing position in the sub-scanningdirection and the BD sensor 215 for detecting the writing start positionin the main scanning direction can be configured to detect the lightbeam emitted from the cylindrical lens 220 of the secondary opticalsystem or can be configured to detect the light beam on the optical pathbefore the cylindrical lens 220 of the secondary optical system.

If the light beam is detected after the cylindrical lens 220 of thesecondary optical system, since the light beam is narrowed by thecylindrical lens 220 in the main scanning direction and the sub-scanningdirection, the power of the light beam per unit area at the sensor isincreased. Therefore, the light beam can be detected if the sensitivityof the BD sensor 216 is relatively reduced.

If the light beam is detected before the cylindrical lens 220 of thesecondary optical system, especially if the light beam is detectedbetween the second fθ lens 203 and the cylindrical lens 220 of thesecondary optical system, since the respective beams emitted from thesecond fθ lens 203 travel in the sub-scanning direction with the opticalaxes thereof parallel to each other, the tolerance of the positioning isincreased for the installation of the BD sensors 215, 216 and the BDsensors 215, 216 can be configured integrally with the unit, which iseffective for the miniaturization of the apparatus or the improvement ofthe installation accuracy (adjustment accuracy of the light beam).Alternatively, a sensor with a relatively low accuracy can be applied tothe BD sensors 215, 216.

As described above, with regard to the installation positions of the BDsensors 215, 216, different effects can be obtained for a case where thesensors are provided before the cylindrical lens 220 of the secondaryoptical system and a case where the sensors are provided after it, andoptimum positions can be selected suitably in accordance with theintended detection accuracy, sensors to be used, or spatial conditionsfor disposing the BD sensors 215, 216. Since the BD sensors 215, 216 canbe configured integrally with the optical scanning unit, theminiaturization of the apparatus or the improvement of the sensorinstallation accuracy (improvement of the adjustment accuracy of thelight beam) can be achieved.

FIGS. 8A to 8C are diagrams for describing a configuration example ofthe BD sensor that can be applied to the embodiment. The configurationof FIGS. 8A to 8C can be applied to both the BD sensor 216 for detectingthe writing position in the sub-scanning direction and the BD sensor 215for detecting the writing start position in the main scanning direction.FIG. 8A shows a configuration example of a typical BD sensor, and the BDsensor 215 (216) includes a light receiving portion 217 composed of aphotodiode with a size of about 2 cm square, for example. When theoptical scanning unit (LSU unit) is manufactured, the unit is assembledsuch that a light beam L is made incident at the center position of thelight receiving portion 217.

When configured as described above, for example, if printing disturbancesuch as printing deviation is generated, the error signal is output whenthe light beam L is almost out of the light receiving portion 217 asshown in FIG. 8B or 8C. That is, if the area of the light receivingportion 217 is large relative to the beam diameter of the light beam L,the detection accuracy is relatively reduced.

In this embodiment, as shown in FIG. 9, a mask 218 is applied to thelight receiving portion 217 of the BD sensor 215 to increase thedetection accuracy. In the mask 218, a slit is formed to open only thenarrower area of the light receiving portion 217. The slit is formed tohave a width of about 1 mm, for example. Because of this slit, when thelight beam L is made incident on the exposed light receiving portion, itis determined that the writing position of the light beam L isappropriate, and when the position of the light beam is slightly movedto be out of the opening of the slit, the light receiving portion 217cannot detect the light beam L and an error is generated. With thisconfiguration, the detection accuracy of the light beam position can beincreased in the BD sensor 215 (216).

FIG. 10 is a block diagram for describing a configuration example of acontrol system of the optical scanning unit.

An LSU controller 301 inputs the image data signal output from an imagememory, etc. of an image processing portion 402 of the image formingapparatus and sends the image data signal to a laser driver circuit (LDDriver) 302 in conformity with the scanning start timing sent from amain body controlling portion 401 of the image forming apparatus tocontrol the lighting of the laser diode (LD) 101.

The LSU controller 301 controls the reference rotation movement of apolygon motor 303 driving a polygon mirror in conformity with thespecification in the main scanning direction of the image formingapparatus. The LSU controller 301 detects the timing of the mainscanning through the reception of the light beam at the BD sensor 215detecting the writing start position in the main scanning direction andoutputs an error signal to the main body controlling portion 401 if anerror is generated. The LSU controller 301 inputs the detection signalof the BD sensor 216 detecting the writing position in the sub-scanningdirection and outputs an error signal to the main body controllingportion 401 if an error is generated. The LSU controller 301 isconstituted by ASIC (application specific integrated circuits).

Description will be made of an adjustment mechanism of the opticalelements in the embodiment. The optical scanning unit of the embodimentincludes several adjustment mechanisms for the optical elements such asmirrors and lenses on the optical path guiding the light beam emittedfrom the laser diode 101 to the photoreceptor drum 3 of each color. Theadjustment mechanisms can adjust angles and positions of the opticalelements relative to the light beams made incident on the opticalelements. The adjustment mechanisms can perform the adjustment suitablyat the time of the assembly adjustment of the optical scanning unit orat a given point in time after the assembly.

The optical scanning unit of the embodiment includes the adjustmentmechanisms for the second mirror 111 and the third mirror 113 of theprimary optical system and the cylindrical lenses 220 of the secondaryoptical system. The mechanisms for adjusting angles and positions areincluded not only in these optical elements but also in the foldingmirror of the secondary optical system, etc. appropriately. For example,the writing start position in the main scanning direction detected bythe BD sensor 215 is adjusted by changing the angle of the final mirrorbefore the cylindrical lens 220 of the secondary optical system.

Description will be made of the adjustment mechanisms of the second andthird mirrors 111, 113 of the primary optical system. In the opticalscanning unit of the embodiment, the angles of the second mirror 111 andthe third mirror 113 of the primary optical system are independently andvariably set. The second mirror 111 and the third mirror 113 function totake partial charge of the optical path adjustment of the light beam inthe main scanning direction and the sub-scanning direction,respectively.

The angle of the second mirror 111 can be adjusted in the direction ofan arrow A shown in FIG. 3. That is, the second mirror 111 is configuredsuch that the reflection optical paths of the four light beams, whichare emitted from the laser diodes 101 and have the parallel optical axesin the sub-scanning direction, are adjustable in the main scanningdirection. Although the configuration for varying the angle of thesecond mirror 111 is not limited, for example, a support member isprovided for rotatably supporting the second mirror 111 (or a frameportion holding the second mirror 111) so that it can be rotated in thedirection of the arrow A and the support member is fixed on the base 120of the primary optical system. The support member is provided with anadjustment screw that contacts with the back side of the second mirror111 to be advanced or retracted in the direction for rotating the secondmirror 111 in the direction of the arrow A. An adjuster can suitablyadjust the adjustment screw to change the tilt of the second mirror 111to adjust the optical path in the main scanning direction of the lightbeam emitted from the second mirror 111. Biasing means such as a springmay be provided for biasing the second mirror 111 such that the secondmirror 111 follows the movement of the adjustment screw when theadjustment screw is adjusted in the direction away from the secondmirror 111.

The angle of the third mirror 113 can be adjusted in the direction of anarrow B shown in FIG. 3. That is, the third mirror 113 is configuredsuch that the reflection optical paths of the four light beams, whichare emitted from the laser diodes 101 and have the parallel optical axesin the sub-scanning direction, are adjustable in the sub-scanningdirection. Although the configuration for varying the angle of the thirdmirror 113 is not limited as is the case with the second mirror 111, forexample, a support member is provided for rotatably supporting the thirdmirror 113 (or a frame portion holding the third mirror 113) so that itcan be rotated in the direction of the arrow B and the support member isfixed on the base 120 of the primary optical system. The support memberis provided with an adjustment screw that contacts with the back side ofthe third mirror 113 to be advanced or retracted in the direction forrotating the third mirror 113 in the direction of the arrow B. Anadjuster can suitably adjust the adjustment screw to change the tilt ofthe third mirror 113 to adjust the optical path in the sub-scanningdirection of the light beam emitted from the third mirror 113. Biasingmeans such as a spring may be provided for biasing the third mirror 113such that the third mirror 113 follows the movement of the adjustmentscrew when the adjustment screw is adjusted in the direction away fromthe third mirror 113.

In the above configuration, the optical path adjustment can be performedwith the second and third mirrors 111, 113 in the main scanningdirection and the sub-scanning direction, respectively, at the positionson the optical path before and after the cylindrical lens of the primaryoptical system.

In another configuration example of the second mirror 111 and the thirdmirror 113, the angle adjustment may be performed not only in either themain scanning direction or the sub-scanning direction but also in boththe main scanning direction and the sub-scanning direction. Thismechanism can be applied to one or both of the second mirror 111 and thethird mirror 113.

In this case, for example, a frame portion is provided for holding thesecond/third mirror 111, 113 such that the second/third mirror 111, 113can be displaced in both the main scanning direction and thesub-scanning direction, and the contact adjustment screw is provided onone point of the back side of the second/third mirror 111, 113. When theadjustment screw is adjusted and the back side is pushed, that portionmay be advanced toward the front side to enable the angle adjustment inthe main scanning and sub-scanning directions as a result. In this case,biasing means such as a spring may also be provided for biasing thesecond/third mirror 111, 113 such that the second/third mirror 111, 113follows the movement of the adjustment screw when the adjustment screwis adjusted in the direction away from the second/third mirror 111, 113.

The adjustment mechanism of the second/third mirror 111, 113 isconfigured such that the adjustment mechanism can be operated from oneside of the unit configured to be the optical scanning unit. This sideof the unit is arranged to be the operational side (front side) of theimage forming apparatus when the optical scanning unit is incorporatedinto the image forming apparatus. With such a configuration, thesecond/third mirror 111, 113 can be adjusted easily from the operationalside of the image forming apparatus.

Description will be made of an example of an angle adjustment mechanismof the cylindrical lens 220 of the secondary optical system. Thelongitudinal direction of the cylindrical lens 220 of the secondaryoptical system must be parallel to the center axis of the photoreceptordrum 3. Secondary optical system unit of this embodiment is providedwith mechanisms that can adjust the angles of the cylindrical lenses 220of the secondary optical system relative to the photoreceptor drums 3.

FIG. 11 is a diagram for describing the angle adjustment mechanism ofthe cylindrical lens 220 of the secondary optical system and is aperspective schematic diagram of the cylindrical lens 220 and theholding mechanism for it. In FIG. 11, a numeral 224 is a frame holdingthe cylindrical lens; a numeral 225 is a rear supporting portion; anumeral 226 is an eccentric cam; a numeral 227 is an adjustment screw; anumeral 228 is a spring member; a numeral 229 is a front supportingportion; and a numeral 230 is an elongate hole provided in the frame. Rindicates the rear side of the apparatus and F indicates the front side(operational side) of the apparatus.

As described above, four photoreceptor drums 3 are prepared forrespective colors of Y, M, C, and K. The cylindrical lens 220 of thesecondary optical system is provided below each photoreceptor drum 3 tomake the light beam converge on the photoreceptor drum 3 for each color.Each cylindrical lens 220 is held within the metal frame 224. To adjustthe angle of the cylindrical lens 220, the metal frame 224 is providedwith the rear supporting portion 225 and the front supporting portion229. Although the cylindrical lens 220 may be directly supported by eachsupporting portion 225, 229 without using the frame 24, if thecylindrical lens 220 is held within the frame 224, unwanted stress isnot applied to the cylindrical lens 220 and the characteristics thereofcan be stabilized.

The rear supporting portion 225 rotatably supports the frame 224 so thatit can be rotated in the direction of an arrow C at the rear side of theimage forming apparatus. Because of the elongate hole 230 formed in theframe 224, the front supporting portion 229 is configured such that asupport shaft of the front supporting portion 229 can be displaced inthe elongate hole 230 and, therefore, the frame 224 can be slightlyrotated around the axis of the rear supporting portion 225.

In the image forming apparatus, the photoreceptor drum 3 is positionedby and attached to a bearing portion on the wall of the rear side of theapparatus. The position displacement of the photoreceptor drum 3 isoften generated when the photoreceptor drum 3 is displaced on the frontside of the apparatus in the sub-scanning direction with the rear sideof the apparatus acting as a fulcrum. The bearing portion of thephotoreceptor drum 3 on the rear side of the apparatus has essentially avery high accuracy and, when the photoreceptor drum 3 is attached, thebearing portion on the rear side is hardly displaced in the sub-scanningdirection.

Therefore, for the cylindrical lens 220 of the secondary optical system,the rear supporting portion 225 is provided in a portion located on therear side of the apparatus for slightly rotating the frame 224 aroundthe axis of the rear supporting portion 225 and, by performing theoperation similar to the operation for adjusting the positiondisplacement generated in the photoreceptor drum 3, the angle can beadjusted relative to the photoreceptor drum 3.

FIG. 12 is a relevant part enlarged schematic diagram for describing theadjustment mechanism for adjusting the angle of the cylindrical lens ofthe secondary optical system. The angle of the cylindrical lens 220 canbe adjusted in the sub-scanning direction by advancing or retracting theadjustment screw 227. A lateral face of the front end of the frame 224supporting the cylindrical lens 220 contacts with the eccentric cam 226.When the adjustment screw 227 is adjusted in the direction of an arrowD, the eccentric cam 226 is rotated in the direction of an arrow E,presses the lateral face of the frame 227, and displaces it in thedirection of an arrow J. Since the frame 224 is rotatably supported onthe rear side of the apparatus, the frame 224 is rotatably displaced, bythe displacement in the direction of the arrow J, around the axis of therear supporting portion 225 on the rear side of the apparatus. Since theeccentric cam 226 contacts with the frame 224, unwanted stress is notapplied to the cylindrical lens 220.

The spring member 228 is installed at a lateral end portion of the frontside of the frame 224 for the cylindrical lens 220 and biases the frame224 toward the chassis 223 of the secondary optical system in thedirection of an arrow G. When the adjustment screw 227 is adjusted inthe direction of an arrow H, the eccentric cam 226 is rotated in thedirection of an arrow I and the frame 224 is displaced in the directionof the arrow G by the biasing effect of the spring member 228.

With such a configuration, the angle of the cylindrical lens 220 can beadjusted in the sub-scanning direction by the adjustment screw 227. Inthis configuration, since the adjustment screw 227 is installed on thefront side of the apparatus (i.e., operating side of the image formingapparatus), the adjustment can be easily performed, as is the case withthe adjustment mechanism for the second/third mirror 111, 113.

Description will be made of an example of the adjustment of the mainoptical elements at the time of assembling the optical scanning unit. Inthe optical scanning unit of the embodiment, the first optical systemand the second optical system are configured as respective units and theoptical scanning unit is constituted by combining the respective opticalsystems.

The first optical system is constituted by disposing each opticalelement on the base 120 made of die-casting such as aluminum, forexample. The positioning accuracy of the optical system is ensured bythe integral configuration on the die-cast base 120. The secondaryoptical system is constituted by disposing each optical element withinthe chassis 223. As shown in FIG. 4B, the die-cast unit of the primaryoptical system is incorporated into the installation position 222 underthe chassis 223 where the secondary optical system is disposed. In theprimary optical system and the secondary optical system, the opticalelements such as lenses and mirrors are configured to be removable.

The primary optical system unit is incorporated such that the opticalelements are disposed on the under side of the die-cast base 120. Thatis, the primary optical system unit is manufactured by disposing theoptical elements on the die-cast base 120, and the obtained unit isincorporated upside down into the predetermined installation position222 within the chassis 223 of the secondary optical system from theunder side (bottom side) of the chassis 223. Since the primary opticalsystem is incorporated upside down from the under side, it isadvantageous that no wiring appears on the side of the optical elementssuch as mirrors, which makes wiring easier.

In the unit configuration of the primary optical system and thesecondary optical system, positioning means such as pedestals of theoptical elements may be created in advance on the die-cast base 120 oron the wall of the chassis 223 to support the changes in the usage ordesign of the image forming apparatus as much as possible. For example,if the specification of the laser diode 101 is changed to replace with adifferent type of the laser diode (e.g., a laser diode provided with twolaser emitting portions for scanning two lines at the same time), thecorrection of the optical path may be needed due to the replacement ofthe laser diode.

The accuracy of the optical elements of the primary optical system isensured by disposing and integrating the optical elements at thepredetermined positions on the die-cast base. In the embodiment, tosupport the changes in the specification as described above, thepositioning means such as a pedestal, a groove, or a supportingprotruding portion are formed in advance at installation position ofeach optical element for the changes in the specification.

When the specification is changed, the base 120 of the primary opticalsystem is removed from the optical scanning unit to remove the opticalelements and the necessary optical elements are disposed at thepredetermined positioning means. The optical elements may be replaced asneeded or only the installation position thereof may be relocated.

With such a configuration, each optical element can be disposedaccurately when the specification is changed, and even if thespecification is changed, the optical elements can be continuously useddepending on the specification, which has a considerable effect oncosts. In the primary optical system, since the light beams are parallellight and the optical axes of the four light beams are parallel to eachother between the collimator lenses 102 and the cylindrical lens 112, anoptical path length can be freely set and the apparatus can flexiblydeal with space restrictions.

The positioning means of the optical elements for the specificationchanges can be applied to the secondary optical system unit. Forexample, if the printing speed (image formation speed) is to be speededup, the diameter of the photoreceptor drum 3 may be increased. Since theoptical path length from the second fθ lens 203 to the cylindrical lens220 is the same in the second optical system for each color of Y, M, C,and K, the position of the cylindrical lens 220 and the position of themirror guiding the light beam to the cylindrical lens 220 must bechanged to support such specification changes. To support suchspecification changes, the positioning means such as a pedestal, agroove, or a supporting protruding portion are formed in advance at anassumed installation position of each optical element for the changes inthe specification. When the specification is changed, the necessaryoptical elements are removed and the necessary optical elements aredisposed at the predetermined positioning means.

With such a configuration, each optical element can be disposedaccurately when the specification is changed, and if the specificationis changed, the optical elements can be continuously used depending onthe specification. Especially in the second optical system, bycontinuously using the expensive fθ lens after the specification change,a considerable effect on costs can be obtained.

FIGS. 13A and 13B are diagrams for describing a shaft configuration forfixing the optical scanning unit at a predetermined position within theimage forming apparatus; FIG. 13A is a perspective schematic diagram ofthe exposure unit 1 viewed from the left of the operational side; andFIG. 13B is a perspective schematic diagram of the exposure unit viewedfrom the right of the operational side. In FIGS. 13A and 13B, a numeral221 a is a first fixing shaft; a numeral 221 b is a second fixing shaft;numerals 231 a to 231 c are fixing members for fixing the first fixingshaft; numerals 232 a and 232 b are engaging portions for engaging withthe second fixing shaft; and BK, BC, BM, and BY schematically show thelight beams of K (black), C (cyan), M (magenta), and Y (yellow),respectively.

The optical scanning unit is constituted by disposing the opticalelements of the second optical system within the chassis 223 and bydisposing the primary optical system unit at a predetermined positionwithin the chassis 223 as described above. In this way, the opticalscanning unit is formed as a unit within the chassis 223 and isconfigured to be removable from the image forming apparatus such as aprinter.

The image forming apparatus is provided with the optical scanning unit(exposure unit) as described above and exposes the photoreceptor drum 3by the optical scanning unit according to the image data. In thisembodiment, since the optical scanning unit is formed as a unit withinthe chassis 223, the optical scanning unit can be attached to anddetached from the main body of the image forming apparatus. For example,in the case of the specification change such as speeding up of the imageformation, the specification change can be accommodated by replacing theoptical elements of the primary optical system or the second opticalsystem or by changing their arrangement, or the optical scanning unititself can be configured to be exchangeable.

As shown in FIG. 4B, to fix the chassis 223 of the optical scanning unitwithin the image forming apparatus, two fixing shafts 221 a, 221 b areattached to the both sides of the chassis 223 of the optical scanningunit. Within the frame of the main body of the image forming apparatus,supporting portions are provided for fixedly supporting each fixingshaft 221 a, 221 b, and the chassis 223 is fixed to the fixedlysupported shafts 221 a, 221 b. That is, the chassis 223 of the opticalscanning unit is held within the main body of the image formingapparatus by the two fixing shafts 221 a, 221 b.

The fixing shafts 221 a, 221 b fixed to the chassis 223 are providedsuch that the longitudinal directions thereof are the same as thedirection parallel to the axis direction of the photoreceptor drum 3(i.e., main scanning direction). By inserting the optical scanning unitfrom the front side to the rear side of the image forming apparatus, theoptical scanning unit is positioned at a predetermined position, and theengaging portions 232 a, 232 b (FIG. 13B) provided on the chassis 223are engaged with the second fixing shaft 221 b, and the chassis 223 isfixed to the first fixing shaft 221 a with the use of the fixing members231 a to 231 c.

FIGS. 14A to 14D are diagrams for describing a mechanism that fixes thefirst fixing shaft 221 a; FIG. 14A is a perspective schematic diagram ofa relevant part of the chassis; FIG. 14B shows first and secondsupporting members disposed on the rear side and the front side of theapparatus; FIG. 14C shows a third supporting member disposed near themiddle of the first and second supporting members; and FIG. 14D showshow the first fixing shaft is engaged with the first supporting memberand the third supporting member. In FIGS. 14A to 14D, a numeral 233 a isthe first supporting member; a numeral 233 b is the second supportingmember; a numeral 233 c is a third supporting member; a numeral 234 is agroove portion of the first (or second) supporting member; and a numeral235 is a groove portion of the third supporting member.

As shown in FIG. 14A, to the left of the chassis 233 viewed from theoperational side, the first to third supporting members 233 a to 233 care provided for supporting and fixing the first fixing shaft 221 a. Thefirst and second supporting member 233 a, 233 b are provided on the rearside and the front side (operational side) of the apparatus,respectively, and the third supporting member is provided therebetween.

The respective supporting members 233 a to 233 c are provided with thegroove portions 234, 235 for supporting the first fixing shaft 221 a.The first fixing shaft 221 a is supported by the groove portions 234,235 and can be moved in the axis direction of the photoreceptor drum 3.When the first fixing shaft 221 a is disposed in the groove portions234, 235 of the respective supporting members, the first fixing shaft221 a is fixed to the respective supporting members 233 a to 233 c byattaching the fixing members 231 a to 231 c as shown in FIG. 13A to therespective supporting members 233 a to 233 c. Since the engagingportions 232 a, 232 b of the chassis 223 are engaged with the secondfixing shaft 221 b, the chassis 223 is fixed to the respective fixingshafts 221 a, 221 b in this way.

Among the supporting members 233 a to 233 c, the supporting memberactually supporting the chassis is the third supporting member 233 c inthe middle. As shown in FIGS. 14B and 14C, a shape of the grooveportions 234 provided in the first and second supporting member 233 a,233 b is different from a shape of the groove portion 235 provided inthe third supporting member 233 c. That is, the first fixing shaft 221 acontacts with the inner wall surface of the groove portion 235 of thethird supporting member 233 c and is supported by the inner wall surfaceof the groove portion 235.

On the other hand, the groove portions 234 of the first and secondsupporting member 233 a, 233 b are configured such that the upper andlower inner wall surfaces are located away from the surface of the firstfixing shaft 221 a. That is, the first fixing shaft 221 a is looselyinserted into the groove portions 234 with upper and lower gaps.

When the first fixing shaft 221 a is supported by the respectivesupporting members 233 a to 233 c, as shown in FIG. 14D, with respect tothe vertical direction, the first fixing shaft 221 a is supported at onepoint of the third supporting member 233 c, and the first and secondsupporting members 233 a, 233 b do not support the first fixing shaft221 a. The first and second supporting members 233 a, 233 b mainlysupport the first fixing shaft 221 a with respect to the horizontaldirection.

That is, with the configuration as described above, the chassis 223 issupported at three points, which are one point at the center part of thefirst fixing shaft 221 a and two points of the second fixing shaft 221b. Since there are three supporting points in this configuration, thethree points define a plane to enhance the stability of the support ofthe chassis 223.

Of course, the first fixing shaft 221 a may be supported at two or moresupporting points and the chassis 223 may be supported at four or moresupporting points. Although the credibility is enhanced because moresupporting points exist in this case, if one or more supporting point(s)of the four or more supporting points is/are displaced from the planeincluding other supporting points, rattling is generated, which is notpreferred for the stability of the support. Means may be added forfinely adjusting the respective fixing shaft 221 a, 221 b in thesub-scanning direction and, by this means, the tilt of the opticalscanning unit can be adjusted in the sub-scanning direction relative tothe photoreceptor drum 3.

Description will be made of an example of a technique for adjusting theposition of each optical element at the time of assembling of theoptical scanning unit. First, the laser diode 101 is attached to thebody tube 107 (including the collimator lens 102 and the aperture 103)and made to emit the light to verify the light beam on an arbitraryscreen. Since the light beam emitted from the body tube 107 is parallellight, the beam diameter on the screen is constant regardless of thedistance to the screen. The above verification is performed for the fourlaser diodes 101 for Y, M, C, and K to check that the parallel light isemitted from all the laser diodes.

The four body tubes and laser diodes 101 are attached to thepredetermined positions on the base of the primary optical system, andthe laser diodes 101 are made to emit the light. The cylindrical lens112 of the primary optical system has been removed. The light beamsemitted from the four laser diodes 101 are projected on an arbitraryscreen to check whether the four beams have predetermined intervals ornot. That is, since the four light beams travel with the optical axesparallel to each other after the emission from the collimator lenses102, the four light beams projected on the screen have constantintervals. In this case, the beam intervals are constant regardless ofthe distant to the screen. By verifying such behaviors, it is checkedthat each of the four beams is parallel light and that the optical axesof the four beams are parallel to each other. When the beam interval ischecked, the distance between the centroids of the light beams may bechecked.

The optical elements such as the cylindrical lens 112 of the primaryoptical system are disposed at predetermined positions and the primaryoptical system unit is incorporated at the predetermined installationposition 222 of the chassis 223 of the secondary optical system. Theemission of the laser diode 101 is performed to check that four emittedbeams converge on the reflection face of the polygon mirror 201.

A screen is placed between the second fθ lens 203 and the cylindricallenses 220 of the secondary optical system to project four light beamsemitted from the second fθ lens 203. It is verified that the four lightbeams are projected on the screen at predetermined intervals in thesub-scanning direction.

Another screen is placed between the second fθ lens 203 and thecylindrical lenses 220 of the secondary optical system to verify thatthe four light beams are projected on the two screens at the same beamintervals. Since the optical axes of the four light beams after beingemitted from the second fθ lens 203 are parallel to each other atpredetermine intervals in the sub-scanning direction, it can be checkedby verifying the beams projected on two screen as described abovewhether the optical systems are in an optimum state or not.

If the four beams are not arranged at the predetermined intervals on thescreen, the primary optical system, for example, the second mirror 111or the third mirror 113 is adjusted such that the light beams behavedesirably.

According to the present invention, effects as follows can be obtained.

According to the present invention, since the optical scanning unit isformed as a unit, the optical scanning unit is detachable to the body ofthe image forming apparatus. For example, the specification changes suchas the speeding up of image formation can be supported by replacing theoptical elements of the primary or secondary optical system or bychanging their arrangement as described above, and the optical scanningunit itself can be configured to be exchangeable. In this way, theoptical scanning unit and the optical element thereof can be shared by aplurality of models or a plurality of specifications of the imageforming apparatus.

Further, according to the present invention, since the primary opticalsystem unit is manufactured by disposing the optical elements on thebase of the primary optical system and the obtained unit is installedupside down at the predetermined installation position inside thechassis of the secondary optical system from the under side (bottomside) of the chassis, the primary and secondary optical system units canbe easily attached/detached to/from each other and no wiring appears onthe side of the optical elements such as a mirror, which makes wiringeasier.

Further, according to the present invention, to support the changes inthe specification of the image forming apparatus, by forming positioningmeans such as a pedestal, a groove, or a supporting protruding portionin advance at an assumed installation position of each optical elementfor the changes in the specification, each optical element can beaccurately disposed when the specification is changed and if thespecification is changed, the optical elements can be continuously useddepending on the usage thereof, which has a considerable effect oncosts. In the primary optical system, since light beams are parallellight and optical axes of four light beams are parallel to each otherbetween the collimator lens and the cylindrical lens, an optical pathlength can be freely set and the apparatus can flexibly deal with spacerestrictions. In the secondary optical system, since an expensive fθlens can be continuously used even after the specification is changed, aconsiderable effect on costs can be obtained.

Further, according to the present invention, with regard to the opticalelements such as a mirror and a lens on the optical path guiding thelight beam emitted from the laser diode to a photoreceptor drum for eachcolor, since the adjustment mechanism is provided which can adjustangles and positions of the optical elements relative to the light beamsmade incident on the optical elements, the light beams can be suitablyadjusted.

Further, according to the present invention, since the adjustmentmechanism is disposed on one side of the unit configured to be theoptical scanning unit, for example, on an operational side (front side)of the image forming apparatus when the optical scanning unit isincorporated into the image forming apparatus, the optical elements canbe easily adjusted from the operational side of the image formingapparatus.

Further, according to the present invention, since the shafts which areparallel to the main scanning direction are provided on the opticalscanning unit and the shafts are fixed by the shaft fixing members, theunit can be attached to the image forming apparatus with lessdisplacement in the sub-scanning direction and high installationaccuracy.

Further, according to the present invention, the chassis of the opticalscanning unit is supported at three points, which are one point near thecenter part of one fixing shaft and two points of the other fixingshaft, and since there are three supporting points, the three supportingpoints define a plane to enhance the stability of the support of theoptical scanning unit.

1. An optical scanning unit that irradiates a polygon mirror with aplurality of light beams emitted from a light source according to imagedata, the optical scanning unit converting the plurality of light beamsto scanning lights by the rotation of the polygon mirror, the opticalscanning unit scanning and exposing a plurality of photoreceptorssimultaneously with the plurality of scanning lights to form latentimages on the respective photoreceptors, the optical scanning unitcomprising: a primary optical system unit including the light source andan optical system that emits the plurality of light beams emitted fromthe light source toward the polygon mirror; and a secondary opticalsystem unit including the polygon mirror and an optical system thatemits the light beams reflected by the polygon mirror toward thephotoreceptors, wherein the primary optical system unit is fittedremovably to the secondary optical system unit and wherein a combinedunit of the primary optical system unit and the secondary optical systemunit is fitted removably to an image forming apparatus comprising thephotoreceptors.
 2. The optical scanning unit of claim 1, wherein theprimary optical system unit includes a first laser diode, a second laserdiode, a third laser diode, and a fourth laser diode, which act as thelight source, wherein the primary optical system unit further includes:a first mirror that reflects the light beams emitted from the second tofourth laser diodes; a second mirror that reflects the light beamemitted from the first laser diode and the light beams reflected by thefirst mirror; a primary optical system cylindrical lens that acts on thelight beams emitted from the second mirror; and a third mirror thatreflects the light beams emitted from the primary optical systemcylindrical lens toward the polygon mirror, and wherein the secondaryoptical system unit includes the polygon mirror, an fθ lens that acts onthe light beams emitted from the polygon mirror, and a secondary opticalsystem cylindrical lens that makes the light beams emitted from the fθlens converge on the surfaces of the photoreceptors.
 3. The opticalscanning unit of claim 2, wherein the scanning direction on the surfaceof the photoreceptor is defined to be a main scanning direction and thearray direction of the plurality of photoreceptors is defined to be asub-scanning direction, wherein the fθ lens is constituted by a first fθlens and a second fθ lens, wherein the individual light beam emittedfrom the polygon mirror is parallel light in the main scanning directionas well as diffused light in the sub-scanning direction with eachoptical axis of the plurality of light beams traveling in thesub-scanning direction in a diffusing manner forming an angle relativeto each optical axis, wherein the first fθ lens and the second fθ lensconvert the individual light beam of parallel light in the main scanningdirection to convergent light approximately converging on the surface ofthe photoreceptor, wherein the second fθ lens converts the plurality oflight beams diffusing in the sub-scanning direction such that theoptical axes of the plurality of light beams become parallel to eachother, and wherein the second optical system cylindrical lens acts onlyon the sub-scanning direction of the light beams emitted from the secondfθ lens to convert the light beams including the individual light beamthat is parallel light and the plurality of light beams which opticalaxes are parallel to each other such that the individual light beamapproximately converges on the surface of the photoreceptor as well assuch that the plurality of light beams approximately converges on thesurfaces of the photoreceptors.
 4. The optical scanning unit of claim 1,wherein the secondary optical system unit is configured by disposing aplurality of optical elements including the polygon mirror within achassis and wherein the primary optical system unit is loaded to thesecondary optical system unit from the bottom side of the chassis wherethe optical elements are disposed.
 5. The optical scanning unit of claim1, wherein at least a part of the optical elements constituting theoptical systems of the primary optical system unit and the secondaryoptical system unit is configured to be removable such that reassemblingcan be performed depending on the performance of an image formingapparatus comprising the photoreceptors.
 6. The optical scanning unit ofclaim 5, wherein the primary optical system unit and/or the secondaryoptical system unit include(s) a positioning mechanism for changing theinstallation positions of the optical elements for reassemblingdepending on the performance of an image forming apparatus and whereinwhen the optical elements are reassembled, unnecessary optical elementscan be removed to dispose necessary optical elements at the positioningmechanism.
 7. The optical scanning unit of claim 1, wherein at least apart of the optical elements constituting the optical systems of theprimary optical system unit and the secondary optical system unit isconfigured to be adjustable in an angle or a position relative to theincident light beams.
 8. The optical scanning unit of claim 7, whereinthe adjustable optical elements include the second mirror, the thirdmirror, and the secondary optical system cylindrical lens.
 9. Theoptical scanning unit of claim 8, wherein an adjusting mechanism foradjusting the angle or position is configured so as to be operated fromone side of each optical system unit.
 10. The optical scanning unit ofclaim 9, wherein the one side of each optical system unit is theoperational side of an image forming apparatus to which the opticalsystem units are attached.
 11. The optical scanning unit of claim 1,wherein a chassis of the secondary optical system unit includes twofixing shafts and wherein the combined unit of the primary opticalsystem unit and the secondary optical system unit is fixed to the imageforming apparatus by the fixing shafts.
 12. The optical scanning unit ofclaim 1 wherein a chassis of the optical scanning unit is supported,with respect to the vertical direction, at three points of the fixingshafts and fixed to the fixing shafts.
 13. An image forming apparatuscomprising the photoreceptors and the optical scanning unit of claim 1,wherein the optical scanning unit forms latent images on thephotoreceptors, the latent images being developed for image formation.14. The optical scanning unit of claim 2, wherein the secondary opticalsystem unit is configured by disposing a plurality of optical elementsincluding the polygon mirror within a chassis and wherein the primaryoptical system unit is loaded to the secondary optical system unit fromthe bottom side of the chassis where the optical elements are disposed.15. The optical scanning unit of claim 3, wherein the secondary opticalsystem unit is configured by disposing a plurality of optical elementsincluding the polygon mirror within a chassis and wherein the primaryoptical system unit is loaded to the secondary optical system unit fromthe bottom side of the chassis where the optical elements are disposed.16. The optical scanning unit of claim 2, wherein at least a part of theoptical elements constituting the optical systems of the primary opticalsystem unit and the secondary optical system unit is configured to beremovable such that reassembling can be performed depending on theperformance of an image forming apparatus comprising the photoreceptors.17. The optical scanning unit of claim 3, wherein at least a part of theoptical elements constituting the optical systems of the primary opticalsystem unit and the secondary optical system unit is configured to beremovable such that reassembling can be performed depending on theperformance of an image forming apparatus comprising the photoreceptors.18. The optical scanning unit of claim 2, wherein at least a part of theoptical elements constituting the optical systems of the primary opticalsystem unit and the secondary optical system unit is configured to beadjustable in an angle or a position relative to the incident lightbeams.
 19. The optical scanning unit of claim 3, wherein at least a partof the optical elements constituting the optical systems of the primaryoptical system unit and the secondary optical system unit is configuredto be adjustable in an angle or a position relative to the incidentlight beams.
 20. The optical scanning unit of claim 2, wherein a chassisof the secondary optical system unit includes two fixing shafts andwherein the combined unit of the primary optical system unit and thesecondary optical system unit is fixed to the image forming apparatus bythe fixing shafts.